The occurrence of white light LED represents a substantive progress of LED from labeling function to lighting function. The light emitted by white light LED most closes to sunlight, and can better reflect the true color of the object under irradiation. From the view point of technology, white light LED is undoubtedly the most advanced LED technique due to its further outstanding features of no pollution, long lifetime, resistance to shock and resistance to impact, and will become a new generation of light source—the fourth generation of electric light source in the 21th century. White light LED will have a very broad range of applications.
At present, the white light LED is mostly achieved in the prior art by the methods of exciting a fluorescent material with a UV light-emitting chip or a blue light-emitting chip. However, these methods are somewhat restricted due to the limitation of the fluorescent material.
For example, patents U.S. Pat. No. 5,998,925, U.S. Pat. No. 6,998,771 and ZL00801494.9 disclose that a blue light-emitting chip is used to excite a cerium-activated rare earth garnet fluorescent material (e.g., Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce, abbreviated as YAG; or Tb-garnet, abbreviated as TAG), and yellow light emitted by the fluorescent material excited by the blue light-emitting chip mixes with a portion of the blue light from the blue light-emitting chip to produce white light. The fluorescent materials used in this method apply great restriction to the application and performance of white light LEDs. Firstly, the excitation wavelength range of such fluorescent materials is from 420 to 490 nm, and their the most effective excitation wavelength range is from 370 to 470 nm, but they are not excited by light of UV light region as well as short wavelength region and green light region of visible light. Secondly, the emission spectra of such fluorescent material powders of the rare earth garnet structure can only reach about 540 nm at the maximum and lack red light component, thereby resulting in a relatively low color rendering index of the white light LED.
For example, patents U.S. Pat. No. 6,649,946, US 20040135504, CN 1522291A, CN 1705732A, CN 1596292A, CN 1596478A, and U.S. Pat. No. 6,680,569 relate to rare earth activated nitride or oxynitride fluorescent materials that can be effectively excited in UV to blue light region. The effective excitation wavelength range of such fluorescent materials is somewhat widened, and emission range can cover from green light to red light, but the luminance of these fluorescent materials is relatively low, and the production cost thereof is relatively high. Thus, there is still great limitation to use these fluorescent materials as commercial LED phosphors.
For example, U.S. Pat. No. 6,351,069 relates to a red light-emitting sulfide fluorescent material, which can be added as a color-compensating component to white light LED, to make up color rendering index and to lower color temperature. However, the luminance of the sulfide fluorescent material is low so that it reduces the luminous efficiency of LED, although it increases the color rendering index. Moreover, said fluorescent material has poor chemical stability and ageing resistance and corrodes a chip, thereby shortening the service lifetime of the LED.
In the abovementioned patents, the methods for making white light LEDs involve the use of violet light to excite two or more phosphors, or the use of blue light-emitting chip to excite one or more phosphors, to produce white light. In particular, the use of several phosphors has a very high requirement on the consistency in application properties, such as chemical stability, luminescent property and ageing property, of different kinds of the phosphors, and the handling of the phosphors in a preparation in packaging industry is strictly restricted.